This invention relates to the evaluation of an oxygen gas sensor performance in a motor vehicle. More particularly, the invention relates to diagnosing sensor performance degradation based on increased response time.
In order to improve the efficiency of an internal combustion engine in a motor vehicle, an oxygen sensor is often used to sense the oxygen content of the exhaust gas, and the air-fuel mixture admitted to the engine is adjusted by the engine management system according to the sensed oxygen level of the exhaust gas.
As the oxygen sensor deteriorates with age, the response time of the oxygen sensor can increase, leading to a less than optimal air-fuel mixture and to reduced engine efficiency. A known method of monitoring the efficacy of the oxygen sensor involves measuring the response of the oxygen sensor when the amount of fuel admitted to the engine is forcibly changed during feedback control, as disclosed in U.S. Pat. No. 5,685,284. The inventors herein have recognised a disadvantage with the above approach. This method is complicated and requires an increased degree of accuracy in the control of the fuel supply.
An object of the present invention is to provide an improved method for evaluating performance of an oxygen sensor.
The above object is achieved and disadvantages of prior approaches overcome by a performance evaluation method for an oxygen sensor that detects an oxygen concentration level in an exhaust gas from an internal combustion engine. The method includes the steps of: cutting off a fuel supply to the internal combustion engine and allowing the detected oxygen concentration level of the exhaust gas to rise; reinstating said fuel supply after the detected oxygen concentration level has risen above a pre-determined upper threshold; measuring a fall time for the detected oxygen concentration level to fall to a pre-determined lower threshold from the moment the fuel supply is reinstated; and producing an oxygen sensor degradation signal if the measured fall time exceeds a pre-set time.
According to a second aspect of the present invention, there is provided an oxygen sensor performance evaluating system that detects an oxygen concentration level of an exhaust gas of an internal combustion engine. The system includes: means for cutting off a fuel supply to the internal combustion engine and allowing the detected oxygen concentration level of the exhaust gas to rise; means for reinstating a fuel supply after the detected oxygen concentration level has risen above a pre-determined upper threshold; means for measuring a fall time from the moment the fuel supply is reinstated for the detected oxygen concentration level to fall to a pre-determined lower threshold; and means for producing an oxygen sensor degradation signal if the measured fall time exceeds a pre-set time.
According to a third aspect of the present invention, there is provided a performance evaluating system for an oxygen sensor that detects an oxygen concentration level of an exhaust gas of an internal combustion engine with an engine management system. The system includes: a microprocessor having a counter-timer and being adapted to receive signals from the engine management system and the oxygen sensor, wherein if the microprocessor receives a command signal from the engine management system indicating that fuel to the engine has been cut off, followed by a signal from the oxygen sensor indicating that the detected oxygen concentration level has reached an upper threshold, the microprocessor is adapted to measure an elapsed time from the moment the engine management system issues a command for fuel reinstatement until the detected oxygen concentration level has fallen to a lower threshold value, and if the elapsed time is greater than a pre-set time, to issue an oxygen sensor degradation signal.
An advantage of the above aspects of the invention is that since feedback control of the fuel supply is not required, the accuracy in the control of the fuel supply is not important in determining whether the oxygen sensor performance is degraded. The upper threshold in the oxygen content after which fuel is reinstated need not be sensed, and may therefore be assumed to have been reached after a pre determined time interval after fuel cut off has occurred, but preferably the oxygen sensor is used to determined when the upper threshold has been reached.
The upper threshold may be the oxygen concentration at which the oxygen sensor saturates, and the lower threshold will typically be fixed at a value between 70% and 85% of the upper threshold oxygen concentration.
However, the lower threshold may be varied as a function of the reinstated fuel level in order to take into account any effect of the reinstated fuel level on the actual oxygen content in the exhaust.
After the upper threshold has been reached, the fuel may be reinstated by the engine management system when the accelerator pedal is depressed, or alternatively the fuel may be reinstated just before the engine speed has dropped to a low enough value for the engine to stall, so that in either case the failure determination method does not interfere with the fuelling of the engine.
To provide reproducible starting conditions, the fall time for the sensed oxygen content to reach the lower threshold may be measured from the moment the engine management system issues a command signal for fuel reinstatement.
The engine management system may provide a command signal for fuel reinstatement that comprises a single step, so that fuel reinstatement is as abrupt as possible.
The fall time may conveniently be measured by a counter-timer that is set to run by a microprocessor when the microprocessor senses the negative edge of the command signal for fuel reinstatement issued by the engine management system.
The counter-timer may be re-set to zero by the microprocessor after the lower threshold has been reached, but preferably the counter-timer will be re-set before fuel reinstatement.
The pre-set time at which the oxygen level fall time is deemed excessive and a degradation signal is produced may be set as a function of the reinstatement fuel level and the value for the lower threshold, but typically, the pre-set time will be fixed at about 2 secondsxc2x120%.
The oxygen sensor degradation signal produced if an excessive fall time is measured may cause a light or other warning device to turn on in order to alert the person operating the engine that the oxygen sensor needs service.
The performance evaluation method may be carried out on board a vehicle as it is travelling, rather than in a garage, for example.
Other objects, features and advantages of the present invention will be readily appreciated by the reader of this specification.